There is no such thing as a bare-faced lie - fibbers are betrayed by slight facial tics, say American police. British bobbies could soon be using the theory for interrogation. By BBC News Online's Megan Lane.

Good fibbers don't just lie through their teeth, but every other part of their body - they spin a line while consciously mimicking open and honest body language.

But even practised liars cannot control tiny facial tics when they are being economical with the truth.

British bobbies will not ape their US counterparts

Police in the UK could soon be taught how to spot these "micro expressions" when quizzing rape and murder suspects.

Their American counterparts already use the technique in the interview room. They call it "analytical interviewing".

Although studying body language is thought a blunt instrument because "trust me" gestures are easy to mimic, analytical interviewing is considered to be a more exact science.

A suspect who is lying may show a very slight pursing of the lips, a momentary raising of the eyebrow or an imperceptible shrug. Such involuntary actions suggest to the interviewer they have hit a nerve.

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But the technique is not failsafe, warns Detective Constable Tony Collins, an interviewing expert from the National Crime Faculty.

"You can't just look at someone's face and tell they are lying. This is just an indicator that something is not right. It shows they're not comfortable with that line of questioning.

"It might not be that they're guilty - it could be something from their childhood."

Research with officers in Germany showed we were rubbish at reading body language

Tony Collins

However, one advantage is that micro-expressions cross cultural boundaries.

"Laughter, anger, fear, disgust, these are generic to everybody - and micro-expressions are a short version of these feelings."

If someone is startled but trying not to show it, an involuntary eyebrow twitch could betray their surprise. If they are trying to suppress a nervous laugh, the muscles around their mouth will tighten.

"If you can detect a subtle movement, it allows the interviewer to explore further. The skill comes in drawing out the truth through good interview techniques."

Only interview specialists would be trained

The practice of reading conventional body language has been widely discredited in crime fighting.

"Research with [police] officers in Germany showed we were rubbish at reading it," says Mr Collins.

"The reason seems to be that there's a culture that [police officers] know everything - maybe because they have seen similar crimes before. They might ask a perceptive question, but lean forward across the desk, thinking they are going to nail a confession.

"The person then leans back because his space has been invaded, and the police officer thinks, 'oh, lying'."

But while analytical interviewing may represent an important step forward in interview techniques, Mr Collins doubts British officers will adopt other policing techniques favoured across the Atlantic.

"Their whole reason for interviewing suspects is to get a confession - ours is to get to the truth."